Like http://bit.ly/fafQa9 goes to http://en.wikipedia.org/wiki/Number_sign#Computing? If you can’t do it directly through Twitter then I guess you’ll have to go via a URL shortening service and paste the link in.

# is a special character in the Twitter world indicating a topic name so putting a # in a URL will cause issues and unintended results as twitter software will interpret that as a hash tag and try to hyperlink it to search results containing that tag. As mentioned above you'll want to use a URL shortener to work around this as it will obviously not have a # in the URL but instead redirect to the proper URL with the # included.